This invention relates to pressure-fluid operated actuators provided with biasing spring means.
In a known actuator (British Pat. No. 1,270,941) a vane is pivotally mounted in a fluid pressure chamber in a casing and a return or bias spring acts on the vane so that it is urged to one end position from which it can be displaced by fluid pressure applied to one side of it. The biasing spring is carried in a housing that is detachably secured to the casing and the spring torque can be adjusted by releasing the housing and rotating it relative to the casing.
This procedure, in actuators of the sizes commonly used for industrial applications at least, is not easy and can be dangerous because of the amount of energy that must be stored in the spring. Control of the adjustment is difficult because of the need to detach the spring housing from the actuator casing and levers must be used to hold the housing steady and to rotate it manually to wind up the spring. These factors bring the risk of injury to the user and damage to the actuator. If adjustment is to be made after the actuator has been installed, difficulty of access may make it necessary to remove it to carry out the adjustment.
It is known to use a worm and wormwheel mechanism to wind up a coiled spring. Such worm drives are disclosed in U.K. Pat. Nos. 320,400 and 290,954. There is also known, from U.K. Pat. No. 271,862, an arrangement for a typewriter carriage movement mechanism comprising a tension spring, which is adjusted through a worm drive from an electric motor. These known worm and wormwheel mechanisms are however relatively expensive to produce.
It is an object of the present invention to provide a worm and wormwheel adjustment mechanism for a spring-biased pivoted vane actuator that can be economically produced.
It is a further object to provide such a mechanism in which the production can be simplified by the use of die-cast or moulded parts that require little or no further machining.